A widely used type of a gas turbine engine is a turbofan. The distinguishing feature of the turbofan is an axial flow fan disposed in the forward portion of the engine within an open duct. The fan is equipped with rotating blades and stationary vanes. Each fan blade comprises an airfoil portion and a dovetail-shaped root portion secured in a fan disk. The fan disk includes a plurality of dovetail-shaped slots disposed circumferentially therein to engage the dovetail root portion of the blades. The disk and the blades are typically manufactured from a metal alloy.
Windmilling is one of many conditions to which the gas turbine engine is subjected, having adverse consequences. During windmilling, which occurs when the aircraft is parked on the ground with its engine shut down, wind enters the forward or aft portion of the engine and imparts tangential (with respect to the disk) motion to the fan assembly, thereby subjecting the fan blades to rocking motion within the disk slots. As the fan blades and disk rotate, a gravitational force tends to pull each blade reciprocally in opposite tangential direction depending upon which side of the engine the blade is disposed. As the blades reciprocate tangentially due to windmilling, they rock from one side of the slot to the opposite side of the slot banging against the opposite side of the slot. Such repetitive impact causes abrasion on the disk and the blades. Although both the disk and the blades are coated with an antigallant coating, both the disk and the blades are nonetheless damaged gradually over extended periods of time.
Although the top portion of the dovetail root portion experiences the most damage due to additional wear caused by the centrifugal loading during operation of the engine, the bottom of the dovetail portion also becomes worn. The problem of abrasion on the bottom of the dovetail is exacerbated by the location of vent openings thereon, which are characteristic of and necessary for the manufacture of hollow fan blades. Furthermore, the vent openings must be sealed to prevent potential contaminants, such as sand, dust, or oil, from entering into internal cavities of the blade. The sealants used to close the vent openings must also be protected from abrasion due to motion of the blades within the slot.
The worn blades and disks either must be repaired periodically or replaced if damage is beyond repair. Both alternatives are costly. The problem of tangential motion in fan blades during windmilling is particularly acute in modern blades that do not include mid-span shrouds. The mid-span shrouds tend to provide structural support and to hold blades separated and in an upright position.
U.S. Pat. No. Re 33, 954 entitled "Rotor Blade Assembly" issued to Honda et al and assigned to United Technologies Corporation discloses a fan assembly having a single wedge-shaped protrusion extending outwardly from the base of each disk slot and a tapered surface at the bottom of the root section of the corresponding blade. Although the arrangement facilitates improved disassembly of blades from the disk, it does not minimize the above noted reciprocating tangential motion of the blades during windmilling.
During operation of the engine, the disk experiences extreme stresses, including a circumferential (hoop) stress. The circumferential stress field in the disk is a substantially circular, continuous field occurring in a "live rim", which is defined as a continuous outer contour within the disk, without any interruptions. The circumferential stress is particularly undesirable if it occurs in locations such as the above-noted wedge-shaped protrusions at the bottoms of the disk slots.
Thus, there is a need to minimize damage to fan blades and to the disk that may be caused by the reciprocating tangential motion of the fan blades within the disk slots without a weight penalty. Additionally, it is necessary to reduce hoop stress proximal to the bottoms of the disk slots.